Parametric devices are flexible and convenient sources of widely-tunable coherent radiation, encompassing all time-scales from the femtosecond pulse to the continuous-wave. In these, a coherent beam of electromagnetic radiation is used to stimulate a non-linear process in a non-linear optical crystal, resulting in the division of the power/energy in the coherent pump wave into two generated waves, typically referred to as the signal and idler waves. The signal is usually defined as that wave providing the useful output, and hence throughout this document is identified as the wave having the longer wavelength of the two generated waves.
Parametric devices can operate in a variety of configurations including amplifiers, oscillators and generators. In a parametric amplifier an intense coherent pump wave is made to interact with the nonlinear optical crystal to produce amplification at the signal and idler optical wavelengths. A parametric oscillator uses a parametric amplifier inside an optical cavity resonant at one or both of the signal and idler waves. Here, the signal and idler waves are either self-starting from noise/parametric fluorescence or the cavity is injection seeded by a suitable source operating at the signal and/or idler wavelength. A parametric generator generates optical waves by the interaction of an intense coherent pump wave with a nonlinear optical crystal to parametrically produce two other optical waves. No cavity is provided for the down-converted waves since parametric gain is sufficiently high as to allow adequate transfer of energy/power to these waves with only non resonant single (or multiple) passing of the pump and or idler and or signal waves through the nonlinear medium. Again, in this case the signal and/or idler waves are either self-starting from noise/parametric fluorescence or the generator is injection seeded by a suitable source operating at the signal and/or idler wavelength.
There is considerable interest in extending the spectral coverage of parametric devices. This is because they are often used as sources of coherent radiation in spectral ranges either not covered by any other sources, or where a single parametric-wave source is capable of replacing a number of sources that would otherwise be needed in order to provide the spectral coverage required. However, a serious limitation of known parametric devices is the detrimental effect of absorption of one or more of the three waves involved in the nonlinear interaction within the nonlinear medium itself. As a result the spectral coverage attainable through a particular parametric generation scheme is often limited only by the presence of absorption and not by the nonlinear or phase-matching characteristics of the nonlinear medium being employed. Elimination of the restriction imposed by absorption would result in improved spectral coverage.
One solution for overcoming problems due to absorption has been identified. This involves using non-collinear phase-matching in such a way as to cause the wave subject to absorption, usually the signal wave, to rapidly walk-out of the nonlinear medium in a direction that is substantially lateral to the propagation direction of the pump wave. Examples of this technique are described in the articles “Efficient, tunable optical emission from LiNbO3 without a resonator”, by Yarborough et al, Applied Physics Letters 15(3), pages 102-104 (1969); “Coherent tunable THz-wave generation from LiNbO3 with monilithic grating coupler”, by Kawase et al, Applied Physics Letters 68(18), pages 2483-2485 (1996), and “Terahertz wave parametric source”, by Kawase et al, Journal of Physics D: Applied Physics 35(3), pages R1-14 (2002).
FIG. 1 is an illustration of the known non-collinear phase-matching process. More specifically, FIG. 1(a) illustrates the geometry of the interacting pump 1, idler 2 and signal 3 waves in the nonlinear medium 4. FIG. 1(b) illustrates the phase-matching process through a so-called k-vector diagram, where kp, ki and ks are the wavevectors of the pump, idler and signal waves respectively, angle θ is the angle subtended by the pump 1 and idler 2 waves and angle φ the angle subtended by the pump 1 and signal 3 waves.
As can be seen from FIG. 1(b), in the known non-collinear phase matching process the pump wave 1 and idler wave 2 are not themselves collinear within the nonlinear medium 4. However, to maintain the necessary nonlinear interaction between them throughout the length of the nonlinear medium 4, they must be of sufficient radial (transverse) extent to maintain an overlap between them throughout the length of the medium 4. This means that it is not possible to employ small (i.e. tightly focused) beam sizes for these waves. Having small beam sizes is desirable because it increases the intensities of the waves, so as to reduce the power or energy necessary for attaining a level of parametric gain required for the operation of the device.